In the manufacture of semiconductor devices, an oxide layer is frequently used for masking during such procedures as etching, solid state diffusion, or ion-implantation, or as a passivation layer to permanently protect the surface of the substrate. These oxide layers are usually deposited by either liquid phase growth, vapor phase growth, controlled pyrolysis, or precipitation (i.e., polycrystalline deposit). A major disadvantage of the liquid phase deposition is that it is difficult to grow extremely uniform thin films over a large area, which is a feature that is highly desirable in certain solar cell and integrated circuit technologies. Vapor phase deposition can be accomplished by physical vapor deposition (PVD), such as sublimation, sputtering, flash evaporation, molecular beam epitaxy (MBE), and by chemical vapor deposition (CVD), which makes use of reactive intermediates. One disadvantage of physical vapor deposition is that it is impractical when incongruent volatilization occurs, as is the case for GaAs.
With the decrease in device dimensions required in advanced integrated circuit technologies, the need arises to exert a tighter control over diffusion profiles in order to maintain relatively small device geometries. This has led to the desire to perform device processing at lower temperatures in order to suppress the movement of dopants and achieve better control of device geometry. Various approaches to this problem in the case of silicon devices have performed processing at one atmosphere pressure but above 1000.degree. C., or at 700.degree. C. and compensated for the lower temperature by the use of high concentration of the gas-phase reactant (H.sub.2 O or oxygen), i.e., at a high pressure. In particular, a substrate such as silicon may be treated with HCl and oxygen (O.sub.2) or trichlorethylene and O.sub.2 at one atmosphere pressure and at a temperature exceeding 1000.degree. C., to form SiO.sub.2, as described by R. E. Tressler, J. Stach, and D. M. Metz in the publication entitled "Gas Phase Composition Considerations in the Thermal Oxidation of Silicon in Cl-H-O Ambients," in the Journal of the Electrochemical Society, Vol. 124, No. 4, 1977, at page 607. A five hour treatment of silicon oriented along the (111) axis with ten percent HCl gas (g) in O.sub.2 (g), as discussed by D. W. Hess and B. E. Deal in a publication entitled "Kinetics of the Thermal Oxidation of Silicon in O.sub.2 /HCl Mixtures," in the Journal of the Electrochemical Society, Vol. 124, No. 5, 1977, at page 735, resulted in oxide layer depositions of the following approximate thicknesses: 1000 angstroms at 900.degree. C., 2000 angstroms at 1000.degree. C., and 3000 angstroms at 1100.degree. C.
In another approach to the problem of surface layer oxidation, water vapor has been used directly as the source of oxygen in the oxidation process, as described by E. A. Irene and R. Ghez, in the publication entitled "Silicon Oxidation Studies: The Role of H.sub.2 O," in the Journal of the Electrochemical Society, Vol. 124, No. 11, 1977, at page 1757. By the process of Irene and Ghez, an oxide layer 2000 angstroms thick was depositied on a silicon substrate by treatment at 893.degree. C. for five hours with N.sub.2 (g) containing 2000 parts per million H.sub.2 O (g). It was also determined by Irene and Ghez that water vapor acts as an accelerator for the oxidation process. For example, treatment of a substrate with dry O.sub.2 (g) at 893.degree. C. for five hours resulted in the deposition of an oxide layer 500 angstroms thick, while treatment of the substrate with O.sub.2 (g) containing 2000 parts per million H.sub.2 O (g) under the same conditions resulted in the deposition of an oxide layer 700 angstroms thick. Further studies by E. A. Irene are reported in the publication entitled "The Effects of Trace Amounts of Water on the Thermal Oxidation of Silicon in Oxygen," in the Journal of the Electrochemical Society, Vol. 121, No. 12, 1974, at page 1613. In this latter work, an oxide layer 300 angstroms thick was formed by treating the substrate at 800.degree. C. for five hours with O.sub.2 (g) containing 25 parts per million H.sub.2 O (g). In addition, Hideo Sunami, in the publication entitled "Thermal Oxidation of Phosphorus-doped Polycrystalline Silicon in Wet Oxygen," in the Journal of the Electrochemical Society, Vol. 125, No. 6, 1978, at page 892, reported that treatment of an undoped silicon substrate at 750.degree. C. for five hours with O.sub.2 (g) saturated with H.sub.2 O (g) produced from H.sub.2 O (liquid) at 90.degree. C. resulted in the deposition of an oxide layer which was 70 angstroms thick, while treatment of a silicon substrate heavily doped with phosphorus (2.2.times.10.sup.21 cm.sup.-3) under the same conditions resulted in the deposition of an oxide layer 500 angstroms thick.
Further, in the publication by Joseph Blanc, entitled "A Revised Model for the Oxidation of Si by Oxygen," in Applied Physics Letters, Vol. 33, No. 5, 1978, at page 424, it was reported that an excellent agreement was obtained with the experimental data on the oxidation of silicon by dry O.sub.2 (g). This model proposes that diffusion through the amorphous oxide is via molecular oxygen, but silicon oxidation occurs through the reaction of a small concentration of atomic oxygen. This model indicates that treatment of a silicon substrate at 893.degree. C. for five hours in dry O.sub.2 (g) would deposit an oxide layer 500 angstroms thick.
Finally, L. E. Katz and L. C. Kimerling, in the publication entitled "Defect Formation During High Pressure, Low Temperature Steam Oxidation of Silicon," in the Journal of the Electrochemical Society, Vol. 125, No. 10, 1978, at page 1680, discussed the oxidation of silicon wafers with steam at a pressure of 300 pounds per square inch at 700.degree. C. for various lengths of time (not stated). The thickness of the oxide layers ranged from 5000 angstroms to 55,000 angstroms. The defect-state concentration after high pressure steam oxidation was found to be nearly identical to the preoxidation condition, and this feature was attributed to the use of a low operating temperature.
While all of the above-mentioned methods have achieved surface layer oxidation at temperatures lower than those previously used in the art, none of these methods has, to our knowledge, been able to achieve a uniform and rapid oxidation rate at a temperature as low as 750.degree. C. under a total pressure as low as one atmosphere.